Hello,

As systems modellers we should not forget the basic
law of limiting factors. Any process which is governed
by multiple factors, like plant growth and development
in this case, it becomes limited by the factor which
is in short supply.

In temperate World where often low temperature is the
limiting factor for plant growth, when all the factors
are near optimum or in abundance, we can get a
relationship between temperature (thermal time
concept) and growth.

But if you consider regions where plant growth is
limited by water supply, then thermal time concept
fails. In such circumstances one can get relationship
between plant growth and water supply. For example in
India where I have some experience, if two plots are
sown on the same day one is irrigated and the other is
rainfed there will be difference in the growth and
also phenology of the crop. In this case the thermal
time is same as they are growing side by side. Here I
am not going into a finer measurement of microclimate
mesurements (Hope Prof.Ritchie is listening).

It would be interesing to hear the comments of
Prof.Joe Ritchie, Prof.Jim Jones and Dr.Hoogenboom in
this regard


Gangadhar Rao Depala
Principal Scientist
Central Research Institute for Dryland agriculture
Hyderabad, India, 500027

--- Derek McNamara <[log in to unmask]> wrote:
> Hello,
> I was just wondering about your comment that light
> and temperature are the
> only factors influencing the timing of physiological
> events.  I have been
> doing a sensitivity analysis of CERES-Wheats soil
> inputs and it appears
> that both the PAW limits and initial soil water
> content have large
> influences on the anthesis date as well as maturity.
>  Am I missing
> something or doing something wrong?  Just wondering.
>
> Derek McNamara
> At 01:55 PM 9/10/02 +0200, you wrote:
> >Nataraj
> >
> >I share your doubts on the concept of thermal time.
> >It looks "ugly" from a scientific point of view,
> >because it has no physiological basis.
> >
> >Yet, the problem is, it seems to work ! And this
> has been
> >proven in numerous studies. In our case, we tested
> the
> >development routine of CERES-Wheat under a wide
> range
> >of environmental conditions in Northern Germany
> >and found close matches between measured and
> modelled
> >data.
> >
> >Nevertheless, despite its wide application, there
> is
> >still confusion, what actually determines the rate
> of
> >development (in case of cereals the plastochron or
> >the phyllochron ?).
> >
> >Other than temperature, a second environmental
> factor
> >influences development - light. You are thus
> absolutely
> >wright in pointing to the fact, that both factors
> (temperature
> >and light) have an interactive influence on plant
> development.
> >
> >Light is not a simple stimulus, because it can vary
> and have
> >influence in several ways, including its spectral
> distribution,
> >quantity, direction, duration and periodicity. Add
> the fact,
> >that different plant organs respond differently to
> these
> >stimuli.
> >
> >Other than temperature and light, there does not
> appear to be
> >another decisive factor influencing the timing of
> physiological
> >events. Doubling of air CO2- concentrations, for
> example, does
> >not influence phasic development or the rate of
> leaf appearance
> >in wheat.
> >
> >Much work needs to be done, until we are actually
> able
> >to formulate mathematical equations, which provide
> >accurate representations of the real system.
> >
> >Until then, we need to stick to the "old" concept,
> >which seems to work fine under practical
> conditions,
> >despite its scientific "ugliness" (which poses the
> >question, whether we should waste our time in
> finding
> >deeper scientific meanings of the concept).
> >
> >In the case of CERES, four out of seven genetic
> >coefficients are dealing with the influences
> >of light and temperature on development
> >(i.e. P1V, P1D, P5 and PHINT). If you work with
> >this model, I would recommend to study the manual
> >and literature on the topic. Proper genetic
> >parameterization is important to achieve good
> >results with the model.
> >
> >By the way, the concept of thermal time is
> >really very old : It was first mentioned in
> >1735.
> >
> >
> >Cheers
> >
> >Matthias
> >
> >--
> >Matthias Langensiepen, PhD
> >Hannover , Germany
> >
> >Internet : www.langensiepen.net
> >Mailbox : [log in to unmask]
>
> Derek McNamara
> Graduate Research Assistant
> Mountain Research Center, Montana State University
> P.O. Box 173490, 106 AJM Johnson Hall
> Bozeman, MT 59717-3490, USA
> 406-994-5073
> [log in to unmask]


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